a Cross-sectional scanning electron microcopy image of CQD bulk homojunction device. b PCE of bulk homojunction devices with different mass ratios of p-type and n-type CQDs. The thickness of the active layer for the n-type CQD devices, p-type CQD devices, and bulk homojunction devices are ~390, 400, and 580 nm, respectively. c Thickness-dependent PCE for bulk homojunction devices and n-type CQD devices (control). Bulk homojunction devices enable the use of much thicker optimized CQD films compared with the case of n-type CQD devices. d Current–voltage characteristics under AM1.5 illumination for n-type CQD device, p-type CQD device, and CQD bulk homojunction device at optimum film thickness. e External quantum efficiency of n-type CQD device and bulk homojunction device at optimum film thickness. The average (symbols) and standard deviation (error bars) are calculated from a sample of six to twelve devices.